nefu 84 http://acm.nefu.edu.cn/test/problemshow.php?problem_id=84

#include<iostream>using namespace std;typedef long long inta;int extend_gcd(inta a,inta b,inta &x,inta &y,inta &gcd){   if(b==0)   {      x=1;      y=0;      gcd=a;   }   else   {      extend_gcd(b,a%b,x,y,gcd);      int temp=x;      x=y;      y=temp-a/b*y;   }}int gcd(int a,int b){   if(b==0)   return  a;   else return gcd(b,a%b);}int main(){   inta n,d,x,y;   int times;   cin>>times;   while(cin>>n>>d>>x>>y)   {      inta a=d;      inta b=n;      inta c=y-x;      inta s=gcd(a,b);       if(c%s!=0)      {         cout<<"Impossible"<<endl;      }       else      {        inta x0,y0,q;        a/=s;        b/=s;        c/=s;        extend_gcd(a,b,x0,y0,q);        x0*=c;        if(b<0)  b=-b;        x0=(x0%b+b)%b;        cout<<x0<<endl;      }   }}

//  和青蛙的約會一樣